Light-sensitive composition and process

ABSTRACT

A photoimageable composition and process for use of the same. The composition comprises a binder that is a mixture of a phenolic resin and a multifunctional epoxy or vinyl compound and a curing system comprising a photoactive compound capable of generating a curing catalyst capable of crosslinking the binder components. The process for use of the composition comprises application of the composition to a substrate, drying of the same, exposing the dried coating to activating radiation, curing the binder in light exposed areas, developing the coating and thermally curing the developed image. The composition is especially useful as a solder mask.

This is a divisional of copending application Ser. No. 07/829,371 U.S.Pat. No. 5,312,715 on Feb. 3, 1992.

BACKGROUND OF THE INVENTION

1. Introduction

This invention relates to light-sensitive compositions. Moreparticularly, this invention relates to aqueous developablelight-sensitive compositions useful as high resolution soldermasks inthe manufacture of printed circuit boards.

2. Description of the Prior Art

Aqueous developable liquid soldermasks are known. They are typicallyapplied as a wet coating, such as by curtain coating, dried, image-wiseexposed to activating radiation, partially cured using heat, developed,such as with an aqueous alkaline solution, and often finally cured.Examples of aqueous developable soldermasks are disclosed in Europeanpatent Publication No. 0,255,989 and in Japanese published PatentDisclosures 55-129341 and 60-26943 published Oct. 7, 1980 and Feb. 9,1985, respectively. Soldermasks disclosed in these publicationstypically consist of at least one alkali soluble phenolic resin, acompound containing at least two epoxy or vinyl ether groups, generallyan epoxy resin, and a photoactive component such as a sulfonium salt oran azide compound capable of initiating crosslinking of the epoxy orvinyl ether compound upon exposure to activating radiation with heatingas necessary. The phenolic component, in adequate concentration, enablesaqueous development, and the combination of the epoxy or vinyl ethercompound and the photoactive component permits cure in exposed areas ofthe film.

In use, image-wise exposure of a dry film of the above composition toactivating radiation initiates a photoreaction, such as the liberationof an acid. The acid catalyzes crosslinking of the epoxy or vinyl ethercomponent of the formulation in imaged areas where the photoreactionoccurs. Selective curing results in areas of differential solubilitythat permit development of the soldermask film.

It has been found that the aqueous developable soldermasks, as disclosedin the referenced publications, fail to yield a high resolution imageupon development. It is believed that a cause of poor resolution is thephenolic component of the formulation. Prior to development, thephenolic component is essentially uncured because the liberation of acidby the photoreaction does not catalyze curing of the phenolic componentof the soldermask. Consequently, upon contact of an exposed film withdeveloper, selective development is possible between exposed andnon-exposed areas because the epoxy or vinyl ether component is cured,but high resolution images are not obtained because the phenoliccomponent is non-selectively dissolved from both exposed and non-exposedportions of the coating alike.

In EPO application publication No. 0,255,989 referenced above, it isdisclosed that a thermal curing agent for the phenolic component may beincluded in the formulation. However, if included, the phenolic of thecomposition cannot be cured prior to development because curing willtake place uniformly in both exposed and non-exposed portions of thefilm making development difficult or impossible. Therefore, inclusion ofa thermal curing agent would not overcome the problem of poor resolutionfollowing development.

SUMMARY OF THE INVENTION

The photoimageable composition of the invention comprises a binder thatis a mixture comprising an alkali soluble phenolic resin and a compoundcontaining at least two active groups selected from epoxy groups, vinylether groups and mixtures of the two, a photoactive compound thatliberates a curing catalyst for binder components in imaged areas uponpatterned exposure to activating radiation, and a crosslinking agent forthe phenolic resin component of the binder that is activated in thepresence of a photogenerated curing catalyst. In a preferred embodimentof the invention, the primary components of the binder comprise apoly(vinyl phenol) and an epoxy resin.

The composition of the invention is used in conventional manner. It isapplied to a substrate, dried, exposed to patterned radiation at awavelength that causes a photoreaction that liberates a curing catalyst,heated as necessary to cure the film in exposed areas, developed andoptionally heat cured. In accordance with the invention, all primarycomponents of the binder, inclusive of the phenolic component, are curedin imaged areas, but not elsewhere. Therefore, upon development, highresolution images are obtained because uncured binder is essentially notsolvated in the exposed areas of the film. Following development, a postdevelopment bake step may be employed to complete the curing reactions.

DESCRIPTION OF THE DRAWING

FIGS. 1, 2, 3a, 3b, 4a, 4b, and 5a are drawing is a diagrammaticrepresentation of comparative processes and results using aphotoactivated curing agent for all polymer components in accordancewith the invention compared to results that would be obtained using anon-photoactive curing agent for the binder components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The phenolic resin component of the composition of the invention is afilm-forming resin having phenolic hydroxyl groups that permitdevelopment of the composition in an aqueous alkaline developer.Phenolic resins are well known in the art. Exemplary phenolic resinsinclude, for example, phenol aldehyde condensates known as the novolakresins, homo and copolymers of alkenyl phenols and homo and copolymersof N-hydroxyphenylmaleimides.

The novolak resins are preferred. They are made following proceduresknown in the art and disclosed in numerous publications such as byDeForest, Photoresist Materials and Processes, McGraw-Hill Book Company,New York, Ch. 2, 1975 and by Moreau, Semiconductor LithographyPrinciples, Practices and Materials, Plenum Press, New York, Chs. 2 and4, 1988, both incorporated herein by reference. Novolak resins are thethermoplastic condensation product of a phenol and an aldehyde. Examplesof suitable phenols for condensation with an aldehyde, especiallyformaldehyde, for the formation of novolak resins, include-phenol;m-cresol; o-cresol; p-cresol; 2,4-xylenol; 2,5-xylenol; 3,4-xylenol;3,5-xylenol; thymol and mixtures thereof. An acid catalyzed condensationreaction results in the formation of a suitable novolak resin which mayvary in molecular weight from about 500 to 100,000 daltons. Thepreferred novolak resins are the cresol formaldehyde condensationproducts.

Poly(vinyl phenol) resins may be formed by block polymerization,emulsion polymerization or solution polymerization of correspondingmonomers in the presence of a suitable catalyst. Suitable materials, forpurposes of the invention, include those materials disclosed in EPOapplication publication No. 0,255,989 and in U.S. Pat. No. 4,439,516incorporated herein by reference.

An additional, though lesser preferred class of phenolic resins forpurposes of the present invention, include homo and copolymers ofN-hydroxyphenyl maleimides. Such materials are also disclosed in theabove-cited European published application 0,255,989 beginning on page2, line 45 and continuing to page 5, line 51, incorporated herein byreference for its teaching of such resins.

Included within the scope of the term "phenolic resin" as used hereinare the copolymers of cyclic alcohols and phenols as disclosed inpublished European patent application No. 0 401 499 having a publicationdate of Dec. 12, 1990 and incorporated herein by reference.

Another component of the composition of the invention is one containingat least two active groups selected from epoxy groups, vinyl ethergroups and mixtures of the two.

Useful epoxy-containing materials are disclosed in the above referencedEPO application publication No. 0,255,989. They may vary from lowmolecular weight monomeric materials to high molecular weight polymersand may vary greatly in the nature of their backbone and substituentgroups. The backbone may be of any type and substituent groups may beany group free of an active hydrogen atom reactive with an oxirane ringat room temperature. Illustrative of suitable substituents includehalogens, ester groups, ethers, sulfonate groups, siloxane groups, nitrogroups, phosphate groups, etc. Exemplary epoxy-containing materialsinclude glycidyl ethers such as the glycidyl ethers of polyhydricphenols obtained by reacting a polyhydric phenol with an excess ofchlorohydrin such as epichlorohydrin. Further examples of epoxymaterials of this type are described in U.S. Pat. No. 3,018,262,incorporated herein by reference.

There are many commercially available epoxy materials suitable for usein the compositions of the invention. Such materials includeepichlorohydrin, glycidol, glycidylmethacrylate, the glycidyl ether ofp-tertiarybutylphenol (e.g. those available under the trade designation"Epi-Rez" 5014 from Celanese); diglycidyl ether of Bisphenol A (e.g.those available under the trade designations "Epon 828," "Epon 1004" and"Epon 1010" from Shell Chemical Co. and "DER-331," "DER-332" and"DER-334, " from Dow Chemical Co.), vinylcyclohexene dioxide (e.g."ERL-4206" from Union Carbide Corp.),3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexenecarboxtlate (e.g. "ERL-4201" from Union Carbide Corp.),bis(2,3-epoxycyclopentyl) ether (e.g. "ERL-0400" from Union CarbideCorp.), aliphatic epoxy modified with polypropylene glycol (e.g."ERL-4050" and "ERL-4052" from Union Carbide Corp.), epoxidizedpolybutadiene (e.g. "Oxiron 2001" from FMC Corp.), flame retardant epoxyresins (e.g. "DER-580," a brominated bisphenol type epoxy resinavailable from Dow Chemical Co.), 1,4-butanediol diglycidyl ether ofphenol formaldehyde novolak (e.g. "DEN-431" and "DEN-438" from DowChemical Co.), and resorcinol diglycidyl ether (e.g. "Kopoxite" fromKoppers Company, Inc.).

Examples of compounds with at least two vinyl ether groups includedivinyl ethers of aliphatic, cycloaliphatic, aromatic or araliphaticdiols. Examples of such materials include divinyl ethers of aliphaticdiols having from 1 to 12 carbon atoms, polyethylene glycols, propyleneglycols, polybutylene glycols, dimethylcyclohexanes, etc. Specificexamples include divinyl ethers of ethylene glycol,trimethylene-1,3-diol, diethylene glycol, triethylene glycol,dipropylene glycol, tripropylene glycol, resorcinol, Bisphenol A, etc.

The photoactive compound used in the composition of the invention is onethat liberates a photogenerated curing catalyst upon exposure toactivating radiation. Preferably, the catalyst is an acid. Thephotogenerated curing catalyst initiates the reactions that cause cureof the binder components in the presence of curing agents, as necessary.Since the curing reactions are dependent upon a photogenerated catalyst,curing takes place only in exposed areas of a coating formed from thecomposition. Since all primary components of the binder are cured in theexposed coating, differential solubility between exposed areas andunexposed areas is excellent resulting in developed images of highresolution without significant loss of binder from exposed areas.

As stated above, preferred photoactive components of the invention arephotoacid generators. Photoacid generators useful in the compositions ofthe invention are known in the art and extensively described in theliterature such as in U.S. Pat. Nos. 4,090,936 and 5,034,304incorporated herein by reference.

One class of preferred acid generators are onium salts of a Group VAelement, onium salts of a Group VI A element, and aromatic haloniumsalts. These complex salts, upon being exposed to activating radiationsuch as ultraviolet radiation or electron beam irradiation, generateacids capable of initiating the required reactions.

Preferred onium photoactive compounds are aromatic iodonium complexsalts and aromatic sulfonium complex salts. These materials are fullydisclosed in the above noted published EPO application No. 0,255,989.

Examples of aromatic iodonium complex salt photoactive compounds includediphenyliodonium tetrafluoroborate, diphenyliodoniumhexafluorophosphate, phenyl-2-thienyliodonium hexafluorophosphate,diphenyliodonium hexafluoroantimonate, di(2,4-dichlorophenyl)iodoniumhexafluorophosphate, di(4-methoxyphenyl)iodonium hexafluorophosphate anddi(3-methoxycarbonylphenyl)iodonium hexafluorophosphate,di(4-acetamidophenyl)iodonium hexafluorophosphate. Examples of aromaticsulfonium compounds include triphenylsulfonium tetrafluoroborate,dimethylphenylsulfonium hexafluorophosphate, tritolysulfoniumhexafluorophosphate, 4-butoxyphenyldiphenylsulfonium tetrafluoroborate,tris(4-phenoxyphenyl)sulfonium hexafluorophosphate,4-acetoxy-phenyldiphenylsulfonium tetrafluoroborate,tris(4-thiomethoxyphenyl)sulfonium hexafluorophosphate,di(methoxynaphthyl)methylsulfonium tetrafluoroborate,dimethylnaphthylsulfonium hexafluorophosphate andphenylmethylbenxylsulfonium hexafluorophosphate.

of the aromatic sulfonium complex salts which are suitable for use inthe compositions of the invention, triaryl substituted salts such astriphenylsulfonium hexafluorophosphate are preferred.

Another class of photoactive compounds suitable for purposes of thepresent invention are the photoacid generators such as those disclosedin U.S. Pat. No. 5,034,304. In particular, halogenated photoacidgenerators are preferred. These materials include 1,10-dibromodecane;1,1-bis[p-chlorophenyl]-2,2-di-dichloroethane; 4,4¹-dichloro-2-(trichloromethyl) benzhydrol or1,1-bis(chlorophenyl)2-2,2-trichloroethane (Kelthane®);hexachlorodimethyl sulfone; 2-chloro-6-(trichloromethyl) pyridine;0,0-diethyl-0-(3,5,6-trichloro-2-pyridyl)phosporothionate (Dursban®);1,2,3,4,5,6-hexachlorocyclohexane;N(1,1-bis[p-chlorophenyl]-2,2,2-trichoroethyl acetamide;tris[2,3-dibromopropyl]isocyanurate;2,2-bis[p-chlorophenyl]1,1-dichloroethylene; and their isomers, analogs,homologs, and residual compounds. Most preferred of the aforesaid istris[2,3-dibromopropyl]isocyanurate.

An additional component of the composition of this invention is a lowtemperature curing agent for the phenolic component of the bindercatalyzed by photogenerated catalyst. Preferred curing agents are acidcatalyzed materials reactive with the phenolic hydroxyl group. Anespecially preferred agent is a melamine formaldehyde resin. Melamineformaldehyde resins are amino resins formed by condensation offormaldehyde with melamine. The resins are typically ethers such astrialkylol melamine and hexaalkylol melamine. The alkyl group may havefrom 1 to as many as 8 or more carbon atoms but is preferably methyl.Dependent upon the reaction conditions and the concentration offormaldehyde, the methyl ethers may react with each other to form morecomplex units.

Melamine resins are known in the art, commercially available fromAmerican Cyanamid Company of Wayne, N.J. under the trade name Cymel anddescribed in American Cyanamid's product bulletin High Solids AminoCrosslinking Agents, published in 1984 as Bulletin No. 4-2111 5K7/84. Inaccordance with this invention, the preferred melamine formaldehyderesin has a degree of polymerization varying between 1.3 and 2.0 andmost preferably, is a member of the Cymel 300 Resin series which arehighly methylated melamine formaldehyde resins. The most preferredmelamine formaldehyde resin for purposes of this invention is Cymel 301which is a hexamethoxymethyl melamine with a low methylol content havingalkoxy groups as the principle reactive groups and a degree ofpolymerization of 1.5.

The preferred catalyzed crosslinking agents are believed to reactprimarily with the phenolic resin. Consequently, it is believed that thefollowing exposure and low temperature cure and prior to development,the photoimaged composition in exposed portions of the film comprise anetwork consisting primarily of epoxy condensation products and thereaction product of the phenolic resin and the melamine resin.

Other conventional additives may be included in the compositions of theinvention such as dyes, fillers, wetting agents, fire retardants and thelike. Sensitizers constitute a preferred additive and are added toincrease the range of wavelength photosensitivity. Suitable sensitizersinclude phenothiazines inclusive of substituted phenothiazines,2-ethyl-9,10-dimethoxy-anthracene, 9,10-dichloroanthracene,9,10-phenylanthracene, 1-chloroanthracene, 2-methylanthracene,9-methylanthracene, 2-t-butyl anthracene, anthracene,1,2-benzanthracene, 1,2,3,4-dibenzanthracene, 1,2,5,6-dibenzanthracene,1,2,7,8-dibenzanthracene, 9,10-dimethoxydimethylanthracene, and thelike.

To formulate the composition of the invention, components are dissolvedin a suitable solvent such as, for example, one or more glycol etherssuch as ethylene glycol monomethyl ether, ethylene glycol monoethylether; esters such as a methyl cellosolve acetate, ethyl cellosolveacetate, propylene glycol monomethyl ether acetate, dipropylene glycolmonomethyl ether acetate and other solvents such as dibasic esters,propylene carbonate and gamma-butaryl lactone.

The concentration of composition components may vary within wide limitsdependent upon the use of the material. Concentration ranges for theprincipal ingredients of the formulation are set forth in the followingtable and expressed as dry solids in parts by weight.

    ______________________________________                                                       Broad Range                                                                            Preferred Range                                       ______________________________________                                        Phenolic Resin   25-65      40-50                                             Epoxy or Vinyl Compound                                                                        15-50      25-35                                             Crosslinking Agent                                                                              5-35       1-20                                             Photoactive Compound                                                                            1-15       2-10                                             ______________________________________                                    

In addition to the above components, other additives commonly found insuch compositions, such as those described above, are present in minorconcentration except for fillers and pigments which may be present inlarger concentration such as, for example, in amounts of from 5 to 30percent by weight of the total of the dry components.

The above components are desirably dispersed in a solvent to form aliquid coating composition. Their concentration in a solvent woulddepend on several factors, for example, the coating method used to applythe material to a substrate. For example, the concentration of the drycomponents in a slot coating composition would be higher than in acurtain coating composition. In general, the concentration of the drycomponents in the solvent may vary from about 10 to 50 weight percent ormore of the total weight of the coating composition. For example, for acurtain coating composition, the percentage may vary between about 20and 30 percent of the total concentration, and for slot coating may varybetween 40 and 50 percent, it being understood that the weightpercentage of solids in the composition would be best determined by theviscosity required for the particular coating method used. If thecomposition is to be applied as a dry film, of course the dry film wouldbe essentially free of solvent.

The coating composition of the invention is used in conventional mannerprovided there is a step of low temperature cure following exposure andpreceding development. Using a method for forming a printed circuitboard for purposes of exemplification, the photoimageable compositionmay be applied to a substrate having a pattern of conductive materialdisposed thereon. The substrate may be a copper laminate substrateprepared by the method described in the second chapter of PrintedCircuits Handbook by Clyde F. Coombs, Jr., 2nd Edition, McGraw-Hill,1979. Other suitable substrates include those prepared by laminatingmultilayer boards for the manufacture of printed circuit boards withvias (through-holes) and interconnections which may contain solder, asdescribed in chapter twenty-three of the above reference, bothincorporated herein by reference.

The composition of the invention is coated onto the substrate usingconventional techniques and preferably, the coating is deposited so asto yield a dry film thickness of at least 0.50 mil on top of a trace asrequired by IPC Specification No. SM-840B. After coating, the wet filmformed is dried at a temperature varying between about 80° and 100° C.for a time of from about 20 to 60 minutes to remove solvent. During thisdrying step, it is desirable to avoid temperatures in excess of 120° C.and drying times of more than 60 minutes to prevent premature curing ofthe composition as this could make development difficult.

The process of transferring an image to the coating involves exposingthe coating to a source of patterned activating radiation to initiatethe photoreaction in exposed areas. Suitable sources of activatingradiation include actinic radiation, x-rays, etc. Following exposure,the layer is subjected to a low temperature bake to initiatecrosslinking of the exposed portions of the binder by the liberation ofthe photogenerated catalyst. The bake conditions comprise heating to atemperature capable of initiating the curing reaction but thetemperature should be below that temperature where thermal crosslinkingwould occur in unexposed areas of the film. Preferably, the baketemperature varies between about 85° to 120° C. and the bake time rangesbetween 1 and 20 minutes. During this step, catalyst generated by thephotoreaction initiates the curing reaction of the binder components inexposed areas of the film.

Following the above procedure, the area not exposed to activatingradiation and free of photogenerated catalyst remains uncured and isreadily dissolved by aqueous alkali solutions such as sodium hydroxide,sodium metasilicate, sodium carbonate, potassium hydroxide, potassiumcarbonate, ethylene diamine and the like. The preferred developer is ametal hydroxide with concentrations between 0.5 to 5% of alkalinehydroxide in water. A typical development time is 30 to 60 seconds.After development of the image, the remainder of the coating ischaracterized by a partial cure whereby most of the binder componentsare crosslinked. A second cure is desirable to achieve full thermal andelectrical solder mask properties. This may be achieved by heating to atemperature of about 120° to 160° C. for a period of time of betweenabout 10 and 60 minutes. The second cure hardens the developed image andthe coating is resistant to soldering and may also be used as apermanent dielectric coating.

The above process is depicted in the drawing, Sequence A, where in step1, substrate 1 is coated with soldermask coating 2. Exposure ofsoldermask coating 2 in step 2, using a photoacid generator for purposesof illustration, results in a region 3 of the coating havingphotogenerated acid in exposed areas as represented by the symbol H⁺.Thereafter, as shown by step 3A, following low temperature cure, coating2 is cured in region 3 to crosslink the exposed areas of the coating.Following cure, the coating is developed to remove unexposed areas ofthe coating with exposed and cured region 3 remaining due to decreasedsolubility in the developer following cure. This is illustrated by Step4A of the drawing. Finally, following full cure, as shown in step 5A,the crosslinking density in region 3 is significantly increased. Bycomparison, with reference to Sequence 2, if an attempt is made to cureall components of the binder of coating 2 in the absence of a catalyzedcrosslinking agent, the result would be a coating 3 where all portionsof the coating would be insoluble in developer as shown in steps 3B and4B where in step 3A, high temperature cure is used and in step 4B,development is unsuccessfully attempted.

The following examples illustrate the invention.

EXAMPLE 1 to 9

The following compositions were prepared by customary mixing procedureswhere the phenolic resin used is poly-(p-vinylphenol) (PVP); the epoxyresin is a bisphenol A epoxy; the melamine is hexamethoxymethylmelamine; the photoinitiator (PI) is triphenylsulphoniumtetrafluoroantimonate; and the solvents are 100% propylene glycolmonomethyl ether acetate (S) or a cosolvent of 50% propylene glycolmonomethyl ether acetate and 50% dipropylene glycol monomethyl etheracetate (Co). The concentration of each component for each example is asset forth in the following table.

    ______________________________________                                        Example No.                                                                              1     2      3   4    5   6    7   8    9                          ______________________________________                                        Component                                                                     PVP        45    40     50  50   40  45   50  50   40                         Epoxy      55    60     50  50   45  40   35  35   45                         Melamine    0     0      0   0   15  15   15  15   15                         P I         4     8      8   6    6   8    4   8    4                         Solvent    S     Co     S   Co   Co  Co   Co  S    S                          ______________________________________                                    

Components were mixed together as specified and additional optionalcomponents (i.e. dyes, sensitizer, wetting agent and filler) were added.The solvent was added to the solids in a one-to-one weight ratio.Precleaned copper clad laminate panels 12"×12") were coated with eachformulation using a draw down technique with a #65 Meier rod and driedfor 30 to 40 minutes at 95° C. The boards were then exposed using thestandard IPC-B25 primary image artwork. After exposure, panels weresubjected to a first cure by heating for about 15 minutes at 95° C.Panels were then developed using an aqueous solution of 15 percentsodium hydroxide. After development, panels were subjected to a secondcure by heating at 140° C. for 60 minutes.

The boards coated with the above compositions were then tested forsurvival in an alkaline copper plating bath (pH>13) for periods of 7hours on two consecutive days for a total of 14 hours exposure to thebath. The boards were evaluated based on three criteria: adhesion, haloor encroachment, and spots. The best indication of how well the boardsurvived is by the number of squares which survived 14 hours in thealkaline copper plating bath without lifting from the copper substrate.Adhesion was measured by counting the squares below the pads on theIPC-B25 pattern. The best results are indicated by the highest number ofsquares remaining on the board after the test.

Halo or encroachment was measured at specific locations, indicated byfour areas of the IPC pattern, around the edges of a developed image.Halo's are indicative of non-adhesion in those areas and result inunsatisfactory electrical performance of the board. Each of the areaswas measured for diameter of the halo and averaged. Because of thespacing between these four areas, a 15 mm encroachment was the worstscore.

The boards were also visually examined for spots in the unimaged area ofthe panel. A 1"×1" template was placed on the film in 5 randomlocations, and the number of spots were averaged. Spots indicate lack ofchemical resistance of the coating. Results are shown in the followingtable where adhesion is in pounds per linear inch:

                  TABLE 1                                                         ______________________________________                                        Example No.                                                                              Adhesion      Halo   Spots                                         ______________________________________                                        1           7            15.0   0.8                                           2           0            15.0   0.3                                           3           2            11.0   0.3                                           4           2            13.8   1.8                                           5          21            1.5    2.0                                           6          21            3.3    1.0                                           7          21            1.8    4.5                                           8          21            1.9    3.8                                           9          21            4.3    3.0                                           ______________________________________                                    

Results show that the presence of melamine resin in Examples 5 to 9significantly contributes to the number of squares that survive the 14hour alkaline copper plating bath, and significantly reduces the size ofthe halo around developed areas. However, poly(vinylphenol) resins usedin combination with melamine resins do not reduce the frequency ofspots. Novolak resins appear to be more effective in reducing spotincidence.

EXAMPLES 10 to 18

In the following examples, the same materials were used except that anovolak resin formed by the condensation of formaldehyde with mixedcresols was substituted for the PVP.

    ______________________________________                                        Example No.                                                                              10    11     12  13   14  15   16  17   18                         ______________________________________                                        Component                                                                     Novolak    50    40     40  45   50  50   45  40   40                         Epoxy      50    60     60  55   35  35   40  45   45                         Melamine    0     0      0   0   15  15   15  15   15                         P I         4     8      4   8    4   8    6   8    8                         Solvent    Co    S      S   Co   S   Co   Co  Co   S                          ______________________________________                                    

Preparation, processing and evaluation were all in accordance with theprocedures of Example 1. The results are shown in the following table.

                  TABLE 2                                                         ______________________________________                                        Example No.                                                                              Adhesion      Halo   Spots                                         ______________________________________                                        10         21            15.0   0.8                                           11          2            15.0   0.3                                           12          1            15.0   0.5                                           13         10            15.0   0.0                                           14         21            2.0    0.0                                           15         21            1.5    0.0                                           16         21            1.0    0.0                                           17         21            1.5    0.0                                           18         21            1.1    0.0                                           ______________________________________                                    

Results show that the presence of melamine resin in formulations 14 to18 improves adhesion, significantly reduces the diameter of any halo andreduces the number of spots on the board.

EXAMPLES 19

The following example comprises the most preferred embodiment of theinvention.

    ______________________________________                                                              Parts by Weight                                         ______________________________________                                        Mixed Cresol Novolak Resin                                                                            45.0                                                  Bisphenol A/Epichlorohydrin Epoxy Resin                                                               30.0                                                  Triarylsulfonium hexafluroantimonate                                                                  4.0                                                   Hexamethoxymethylmelamine                                                                             15.0                                                  Filler (talc)           35.0                                                  Additives such as dyes and photosensitizer                                                            3.5                                                   Propylene glycol methyl ether acetate -                                                               100.0                                                 Dipropylene glycol methyl ether acetate                                       mixed solvent                                                                 ______________________________________                                    

While particular embodiments of the invention have been described in theabove Examples, it will be understood that the invention is not limitedthereto since various modifications may be made.

We claim:
 1. A method for forming an imaged film over a substrate, saidmethod comprising the steps of:a. coating said substrate with acomposition comprising a binder and a curing system for components ofsaid binder, said binder comprising an admixture of a phenolic resin anda compound having at least two reactive functionalities selected fromthe group of an epoxy radical, a vinyl ether radical and a mixture of anepoxy and a vinyl ether radical, and said curing system comprising acrosslinking agent for the phenolic resin and a photoactive compoundcapable of generating a curing catalyst for said binder, said phenolicresin being present in an amount sufficient to render unexposed portionsof the composition alkali soluble, said compound having at least tworeactive functionalities being present in an amount capable of hardeningsaid composition and said curing system being present in an amountcapable of curing the components of the binder upon exposure toactivating ratdiation and baking before development; b. exposing atleast a portion of said coating to activating radiation to generate saidcuring catalyst in exposed portions of said coating; c. curing saidexposed portions of said coating by baking the same; and d. developingsaid coating by contact with an aqueous based developer.
 2. The methodof claim 1 where the phenolic resin is selected from the groupconsisting of a novolak resin and a polyvinyl phenol, the compoundhaving the reactive functionality is an epoxy resin, and the curingsystem is an onium salt and an acid activated crosslinking agent for thephenolic resin, each being present in a concentration in parts by weightas follows:

    ______________________________________                                        phenolic resin   25-65                                                        epoxy resin      15-50                                                        crosslinking agent                                                                              5-35                                                        onium salt         1-15.                                                      ______________________________________                                    


3. The method of claim 2 where said components are present in thefollowing amounts in parts by weight:

    ______________________________________                                        phenolic resin   40-50                                                        epoxy resin      25-35                                                        crosslinking agent                                                                              1-20                                                        onium salt         2-10.                                                      ______________________________________                                    


4. The method of claim 2 where said crosslinking agent is a melamineresin.
 5. The method of claim 4 where the melamine resin ishexamethoxymethylmelamine.
 6. The method of claim 2 where said oniumsalt is a sulfonium compound.
 7. The method of claim 2 where thecomposition is applied as a liquid coating composition and theconcentration of solids in solution varies between about 10 and 50percent by weight of the total composition.
 8. The method of claim 2where the composition is applied as a dry film.
 9. The method of claim 2where the step of curing prior to development is by heating to atemperature of between about 85° and 120° C.
 10. The method of claim 2including a post development thermal cure by heating the developedcoating to a temperature of between about 120° and 160° C.